In above shown figure, a ball of mass m strike the wall with velocity $V$ and rebounds back with same velocity. Calculate the change in momentum.

Initial momentum $=mV$ and after rebound from the wall the final momentum $=-mV$  (negative sign is due to V is opposite to initial V )

So change in momentum $=-mV-(mV)=-2mV$ 

The negative sign indicates it is directed to  left.